Generally, prior art electrostatic transducer devices, whether actuators or sensors, comprise first and second members connected to a drive voltage-source. The resulting attractive electrostatic forces cause at least one of the members to move towards the other. For this purpose, the members are normally made of conductive material, such as metals and doped-polysilicon to facilitate charging and discharging and ensure proper operation.
Prior art devices are disclosed, for example, in the “Micromachined Transducers Sourcebook” by G. T. A. Kovacs, WCB/McGraw-Hill, 1998, at pages 278 to 281, such as the classic cantilever actuator having a movable electrode suspended above a stationary counter-electrode by a gap. This suffers from the inextricable link between the gap length and useful deflection, with the latter usually much less than but not exceeding the gap itself. Larger deflections require larger gaps and consequential large voltages that are very often incompatible with standard IC drive electronics. For many applications it is highly desirable to make devices having deflections larger than the actual gap. This book discloses a comb-drives actuator which has a large number of fine interdigitated fingers producing attractive forces mainly due to fringing fields, which can produce larger movements inherently in the substrate plane. Comb-drive actuators suffer from several problems including: relatively large support/springs passive area, limited out-of-plane movement, difficulty in maintaining the desirable centrally-balanced finger positions, particularly at smaller gap widths.
Other prior art includes the article “Distributed Electrostatic Micro Actuator” by Motoharu Yamagauchi et al of Tohoku University published by IEEE in 1993, and U.S. Pat. No. 5,206,557. Both documents provide large number of members connected in series to produce larger in-plane strokes that are not inherently capable of out-of-plane motions. The stacked members effectively form long chains with normal pulling forces. Together with comb-drives, manufacture of these devices is compounded by the requirement to micromachine deep structures with narrow gaps, a difficult task likely to cause trade-off between achievable gap widths and drive voltage levels (and performance). This reduces compatibility with standard integrated circuit drive voltage levels and manufacturing.